It is necessary, but not enough, for entrepreneurs to have a transformative idea. Life has to deal them a chance to act on it. Our anniversary list of the 25 most transformative leaders, icons and rebels of the past quarter-century has no better example of that than Michael Bloomberg.

In 1981, Bloomberg was a rising Wall Street star as head of equity trading and then systems development at Salomon Brothers. When the securities firm merged with commodities trader Phibro, he was one of half a dozen general partners laid off. His severance, $10 million in cash and convertible bonds, doubled his net worth. The day after he left Salomon, Bloomberg used $4 million of that to launch a financial information company called Innovative Market Systems.

Michael Bloomberg gets fired, changes Wall Street

He met financial traders' need for fast, accurate data and analytics, and in doing so swung the balance of power between Wall Street's buy and sell sides.

His belief was that there was an untapped market for transparent price quotes, particularly for bonds, and for accurate financial data and analytics delivered quickly to the desks of Wall Street professionals via dedicated terminals. His first customer was Merrill Lynch. The brokerage bought 22 of Bloomberg's units and invested $30 million for 30 percent of his start-up.

By 1987, when it was renamed Bloomberg LP, it had installed 5,000 terminals at financial services companies. Two years later, it bought back one-third of Merrill's stake for $200 million, giving the company a valuation of $2 billion. Bloomberg was in the process of amassing a fortune that Forbes estimates at $32 billion—making him one of the 20 richest people in the world.

He owns 88 percent of Bloomberg LP, which now has more than 320,000 terminal subscribers. It generated $8.3 billion in revenue in 2013, and commands a third of the $25 billion market for financial data and analytics. Since 2007, it has been nibbling away at the market share of its main rival, Thomson Reuters.

As Richard Goldberg wrote in "The Battle for Wall Street," Bloomberg "became phenomenally wealthy because he was in the truest sense of the word a visionary. He saw what Wall Street wanted and created the means to provide it." He also fundamentally shifted the balance of power between the Street's buy and sell sides, leveling the playing field for the buy side.

Over the years, Bloomberg LP has added a news service, as well as media businesses encompassing TV, radio, magazine and digital outlets. But the terminals remain the center of the enterprise.

Two months after 9/11, Bloomberg was elected mayor of New York in a close race. A lifelong Democrat, he switched his party registration to run as a Republican in the overwhelmingly Democratic city. He won a second term in 2005. For his third term, secured after his aggressive campaign to raise the city's term limits to three from two, Bloomberg ran as an independent on the Republican ballot line.

Since leaving City Hall early this year, Bloomberg has returned to his company but devotes much of his time to his philanthropy and to promoting the issues he cares about—notably public health, education and gun control. An Anglophile, he is building an international hub for his company and foundation in London. The Norman Foster-designed complex, called Bloomberg Place, is due to open in 2016. "It's not a timid building," The New York Times reported Foster as saying.

Much like his patron.

Michael Bloomberg: Lifelong highlights

Appointed U.N. secretary-general's Special Envoy for Cities and Climate Change in 2014

Funds a super PAC that has spent millions on issues he supports, including gun control and efforts to control climate change

Owns more than 10 homes, including properties in Manhattan and New York State's Westchester County and Hamptons; Vail, Colo.; Bermuda; and London